What is the letter U called when it says the /w/ sound in words like suede and penguin? I've read that y and w are semivowels but the U in suede and penguin doesn't really conform to the definition of a semivowel, and experts don't recognise it as a semivowel, so I was wondering if there was another category that I don't know about. I'd like to be able to explain why the U says /w/.
Words like "vowel" and "consonant" and "semivowel" properly refer to the sounds of English.
Now, the way that English spelling works, there's some correlation between letters and sounds; there are five letters that are usually involved in representing vowel sounds, twenty letters that are usually involved in representing consonant sounds, and one letter that can't make up its mind ("sometimes Y"). So it's quite common to talk about "a silent vowel" or "the five vowels", even though those phrases make no sense in the context of English speech. This is absolutely fine; but you shouldn't let it deceive you about what it means to be a "vowel": the letters are "vowels" only because they usually represent vowel sounds.
Also, many people have a rather fuzzy understanding of this fact, so even when people are specifically trying to explain vowels and consonants, they very often trip themselves up by focusing on the letters instead of the sounds. (I'm not sure, but I think this happens firstly because writing is more tangible than speech — it's easier to see the two separate letters in <th> than the two separate sounds in "U" /juː/ — and secondly because we learn about writing explicitly in school, so we have a better conscious understanding of it and a better vocabulary for talking about it.)
Once you understand that, I think the answer is obvious: in suede and penguin, the letter <u> is still the same letter it always is, so by convention it's still (misleadingly) called a "vowel", even though it's not actually representing a vowel sound in these words. So, for example, if someone asks how many vowels are in the word /sweɪd/, what they usually really mean is, how many "vowel letters" are in the written word <suede>; and the answer is three.